Core Java

When to Call System.out.flush() in Java

In Java, System.out.flush() is a method used to force the output stream (System.out) to immediately write any buffered data to the console. When standard output is used, data is often buffered for efficiency. Let us delve into a practical approach to understanding the Java System Out Flush.

1. Understanding System.out.flush() in Java

In Java programming, the System.out.flush() method plays a crucial role in managing output streams. Standard output, represented by System.out, often uses buffering for efficiency. This means that the data is not immediately sent to the console but rather stored in a buffer until certain conditions are met.

The flush() method comes into play when immediate data display is required. It instructs the output stream to forcefully write any buffered data to the console, ensuring timely information presentation. This method proves valuable in scenarios where real-time data display is essential, preventing delays caused by the buffering mechanism. For instance, in applications requiring constant monitoring or where synchronization between the program execution and console output is crucial.

It’s important to note that System.out.flush() is particularly useful in maintaining consistency between the program’s execution flow and the displayed information. Without it, delays in output may occur, leading to a less responsive user experience.

1.1 When to Call System.out.flush() in Java

The decision of when to call System.out.flush() in Java is crucial for maintaining a responsive and dynamic user interface. Here are some scenarios where calling this method is beneficial:

  • Real-Time Progress Updates: When your program involves time-consuming computations or tasks with iterative progress updates, calling System.out.flush() after each update ensures that the information is immediately displayed. This is particularly useful for creating responsive and interactive user experiences.
  • User Interface Feedback: In applications with graphical user interfaces (GUIs), console outputs may be used to provide feedback to the user. Flushing the output stream becomes essential to keep the user informed about ongoing processes, preventing delays in the display of critical information.
  • Monitoring and Logging: In scenarios where your Java program logs events or monitors specific conditions, using System.out.flush() can be valuable. It ensures that log entries are immediately written to the console, helping in real-time monitoring and debugging.
  • Interactive Console Applications: For console-based applications that require user interaction, calling System.out.flush() is essential after displaying prompts or messages. It provides a smoother interaction by avoiding delays in message display, making the application more user-friendly.

2. Working Example

Consider a scenario where you have a program that performs some time-consuming computations and periodically outputs intermediate results. Without flushing the output stream, the results might not be immediately visible. Let’s use System.out.flush() to ensure timely display.

package com.javacodegeeks;

public class FlushExample {

    public static void main(String[] args) {
        System.out.println("Starting the computation...");

        for (int i = 1; i <= 5; i++) {
            // Flushing the output stream to display results immediately

        System.out.println("Computation complete!");

    private static void performComputation(int iteration) {
        System.out.print("Iteration " + iteration + ": ");
        // Simulating a time-consuming computation
        for (int j = 0; j < 3; j++) {
            try {
                // Introducing a delay to simulate computation time
            } catch (InterruptedException e) {
            // Flushing the output stream to display dots immediately

        System.out.println(" Done");

2.1 Explanation

  • Starting the Computation: The program starts by printing a message indicating the beginning of the computation.
  • Performing Computation: The performComputation method simulates a time-consuming computation for each iteration. It prints a message with dots to represent progress.
  • Flushing the Output Stream: After printing each iteration’s progress, System.out.flush() is called to flush the output stream. This ensures that the dots are immediately displayed on the console rather than being buffered.
  • Completing the Computation: After all iterations are complete, a final message is printed to indicate the end of the computation.

2.2 Output

The program produces output like the following:

Starting the computation...
Iteration 1: ... Done
Iteration 2: ... Done
Iteration 3: ... Done
Iteration 4: ... Done
Iteration 5: ... Done
Computation complete!

By flushing the output stream after printing the progress for each iteration, we ensure that the dots are immediately visible during the computation rather than being held in the buffer until the entire loop is completed. This improves the user experience, especially in scenarios where real-time progress updates are essential.

3. Conclusion

In Java programming, the System.out.flush() method emerges as a valuable tool for managing output streams, ensuring efficient and timely information presentation. Through the example discussed, we explored its application in real-time scenarios where an immediate display of progress updates is crucial. The ability to flush the output stream, especially when dealing with time-consuming computations, enhances the user experience by providing continuous feedback. Whether monitoring a process or updating a user interface, System.out.flush() proves instrumental in maintaining synchronization between program execution and console output. As a best practice, incorporating System.out.flush(), when needed, helps prevent delays caused by output buffering, contributing to a more responsive and user-friendly application. Its simplicity belies its significance in scenarios where real-time communication with the user is paramount. In essence, understanding and judiciously using System.out.flush() can greatly improve the responsiveness and effectiveness of Java programs, especially those requiring dynamic and immediate updates to the console.


An experience full-stack engineer well versed with Core Java, Spring/Springboot, MVC, Security, AOP, Frontend (Angular & React), and cloud technologies (such as AWS, GCP, Jenkins, Docker, K8).
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